Hey gents, I'm working on a cab sim and headphone amp in one project and I am looking for a little help. Specifically, I am using Lart's Simple Cabsim into the Ruby amp setup for headphones. One thing I noticed was these both have JFET input buffers (the cabsim is a J201 and the Ruby is an MPF102). I'm trying a through hole layout before resorting to SMD, but I was wondering if I could maybe just swap out the two discrete JFETS for a dual opamp IC with the two buffers being provided by the opamps. I'm not concerned with the Ruby so much, but the topology of the cabsim has me wondering if an opamp buffer would work there. Specifically, it's the filtering off the source of the J201 in the cabsim. I don't know if I can just swap out the J201 with the input impedance resistors and source resistor and leave everything else the same. What do you think?
http://www.muzique.com/lab/buffers.htm
Quote from: Ripthorn on November 26, 2019, 09:58:47 PM
I was wondering if I could maybe just swap out the two discrete JFETS for a dual opamp IC with the two buffers being provided by the opamps.... I don't know if I can just swap out the J201 with the input impedance resistors and source resistor and leave everything else the same.
After reading what Rich (lazily) suggested, you'll be able to discerne buffer building blocks so any equivalent substitution will be easy enough.. :icon_wink:
P.S.
(for argument's shake..)
Although J201 input resistors DO set imput impedance, they are mainly placed there for another more basic reason.. :icon_wink:
May have seemed like a lazy approach but given we haven't yet seen the circuit it's hard to predict any changes.
I thought of it as encouraging the OP to make better informed desicion and maybe learn something. Never too sure if this is necessary I'm not good at gauging sometimes.
No offense meant.
Quote from: Ripthorn on November 26, 2019, 09:58:47 PM
I was wondering if I could maybe just swap out the two discrete JFETS for a dual opamp IC with the two buffers being provided by the opamps.
I think you definitely could. The circuits Rich posted will get you there, but if you get stuck post a schematic of what you've got so far and we'll
mercilessly rip your efforts to shreds provide useful feedback and helpful suggestions ;)
Sorry for the slow response, was in Grand Canyon national park all day :). I have read the buffers section on muzique multiple times, but here is what I am talking about. So here is the original schematic:
(https://live.staticflickr.com/65535/49135085393_5f03d16eb6_h.jpg) (https://flic.kr/p/2hRUa3H)
And here is what I think it should be, but you can see that I am not certain if the source resistor from the JFET buffer in the original is supposed to be pulled out. I pulled it out in the below schematic snippet, does this look correct (keeping in mind this is just the section up until the PNP BJT)?
(https://live.staticflickr.com/65535/49135768902_6b73d379fc_b.jpg) (https://flic.kr/p/2hRXEem)
Looks ok to me. Try it. The source resistor is unnecessary as soon as it's an op-amp not a JFET.
Without messing up with the rest of the circuit, Yes..!!
(as far as R1=R2=1M or greater ..)
For more "noise immune" buffer, you may lower enough R1 & R2 values (10k say) and interpose a 3rd resistor between their junction point & op-amp non-invertin input (pin3)..
Of course, C1 has to go straight on pin3..
P.S.
I leave the suggestion of placement a 10-47 μF cap between R1 & R2 junction point and GND for Rich.. :icon_wink:
(to prove that he didn't mean no offense..) :icon_redface:
edit: Tom is by far faster...!! :icon_twisted:
Quote from: antonis on November 28, 2019, 05:46:01 AM
P.S.
I leave the suggestion of placement a 10-47 μF cap between R1 & R2 junction point and GND for Rich.. :icon_wink:
(to prove that he didn't mean no offense..) :icon_redface:
The placement of a 10-47uf power smoothing cap would be + side at the node of R1, R2, - side to gnd.
Just like the noiseless biasing at the top of the page I linked.
If I understood it correctly.
In the tortoise and the hare. I'm defo a tortoise. ;D
Thanks, guys! I'll try bread boarding it and see what happens.